The more precise this promise is, the easier it is for you as a user of the product to understand if the product will be able to fulfill the promise. It is, at its core, a matter of trust.

We like and trust people who keeps their promises and we likewise like and trust products that do the same.

So make sure the product or company you’re building has a clear and distinct promise directed to a clear and distinct audience. That way it will be much easier for you to fulfill the promise (it should preferable be only one) and make your users like and trust you.